Basal-like breast cancers (BLBCs) express genes characteristic of myoepithelial/basal cells in the normal mammary gland and comprise up to 25% of all breast cancers. Chemotherapy is the only systemic therapy for BLBC, which underexpresses estrogen receptor (ER), progesterone receptor, and HER-2. It preferentially affects younger women and African-American women, and is associated with high histological grade, aggressive clinical behavior, and a high rate of metastasis to the lung and brain. Unlike other breast cancer subtypes, there is no correlation between tumor size and lymph node metastasis. To date, little is known about the molecular basis of BLBC. We have found that the transcription factor FOXC1, which is involved in embryonic development, is consistently and exclusively induced in BLBC and is associated with poor overall survival. FOXC1 overexpression in normal breast epithelial cells and breast cancer cells increased cell proliferation, migration, invasion, and protein levels of NF-kB p65 and ?-catenin. We therefore hypothesize that FOXC1 is a critical marker and functional regulator of BLBC via modulating NF-kB activation and ?-catenin signaling. In addition, we hypothesize that expression of FOXC1, repressed by ER but elicited by ERK/Notch signaling, determines the propensity of BLBC to metastasize to the brain and lung. In Aim 1, we will determine whether FOXC1 regulates cell growth, migration, invasion, and stem cell-like properties of BLBC cells by activating NF-kB and ?-catenin signaling. FOXC1 overexpression and knockdown cell models will be used to examine the mechanisms and effects of the upregulation of p65 and ?-catenin by FOXC1. In Aim 2, we will determine whether FOXC1 regulates BLBC development in vivo and predisposes breast cancer to brain and lung metastasis. The MMTV-FOXC1 transgenic mouse model will be used to investigate the role of FOXC1 in BLBC development. A brain-seeking xenograft model and the metastatic 4T1 syngeneic mouse mode will be used to study whether FOXC1 governs preferential BLBC metastasis to the brain and lung, respectively. In Aim 3, we will define the molecular mechanisms responsible for upstream regulation of FOXC1 overexpression in BLBC. We will investigate whether EGF induces FOXC1 expression in BLBC cells through downstream ERK and Notch signaling, and whether Notch and ER mediate BRCA1 inhibition of FOXC1 transcription. BLBC is a poorly understood area that is of paramount importance in breast cancer. By providing insight into the biological mechanism for BLBC, this study will facilitate establishing a critical functional marker for detection and diagnosis of BLBC. Results may warrant development and clinical investigation of agents that block FOXC1. Finally, understanding how FOXC1 expression is regulated may allow development of FOXC1-based strategies to prevent BLBC. PUBLIC HEALTH RELEVANCE: Basal-like breast cancer is a subtype of breast cancer which has the worst prognosis and is poorly understood. Chemotherapy is the only systemic treatment for these breast cancers. Our study will help establish a critical molecular marker for detection and diagnosis of basal-like cancers, and also help uncover the biological basis for the cellular traits and phenotypes of basal-like breast cancer.